A cell stack of a fuel cell is constituted by a plurality of stacked cell blocks and a plurality of cooling plates each formed with a cooling path and each interposed between adjacent ones of the cell blocks. Each of the cell blocks comprise a plurality of unit cells, and each unit cell is constituted by an electrolyte layer carrying electrolyte, a pair of porous fuel and oxidizing electrodes sandwiching the electrolyte layer therebetween, and a pair of plates provided on the respective outsides of the electrodes for supplying the electrodes with a fuel gas and an oxidizing gas for performing reaction. A manifold pipe that branches from an inlet cooling main pipe is arranged at the interposed cooling plates and is provided with small pipes communicating with the respective paths of the cooling plates. The respective paths of the cooling plates communicate at their outlet sides with an outlet cooling main pipe through small pipes and a manifold pipe similar to those at the inlet sides of the same.
Accordingly, heat generated by the operation of the fuel cell is removed to maintain the fuel cell at its running temperature by cooling water which is used as a cooling medium. The cooling water flows from the inlet cooling main pipe into the paths of the cooling plate and is discharged through the outlet cooling main pipe.
To perform such cooling, there have been known a pressure water cooling system and a boiling water cooling system. Compared with the boiling water cooling system, the pressure water cooling system requires a large quantity of cooling water and is less efficient because it requires auxiliary driving power. Accordingly, the boiling water cooling system has been used more often. The boiling water cooling system utilizes the evaporation latent heat of cooling water, so that only a small quantity of cooling water is required and the auxiliary driving power can be reduced.
Because the cooling plates of the cell stack are vertically stacked, however, the cooling water flowing in the respective cooling plates is different in pressure from each other due to the head difference of the cooling water produced by the difference of height of the respective cooling plates. Consequently uniform boiling cannot be obtained. Accordingly, there is a disadvantage that the respective cooling plates differ in cooling performance from each other. The problems will be further described by reference to the drawings.
In FIG. 7, a plurality of cooling plates, for example, cooling plates 1a, 1b and 1c are interposed among cell blocks to be stacked to a height of several meters. Cooling water is caused to flow through the cooling plates 1a, 1b and 1c in the direction of an arrow P from pipes 2a, 2b and 2c which branch from a pipe 21. The water is discharged through an outlet cooling main pipe 22 in the direction of an arrow Q. In this case, the cooling plates 1a, 1b and 1c are arranged in the highest, in the middle, and in the lowest positions, respectively. Accordingly, the pressures of the cooling water at the respective inlets of the cooling plates 1a and 1c produce a head difference H corresponding to the height of the stack.
FIG. 8 is a graph showing the pressures of the cooling water when the cooling water is caused to flow in the cooling plates 1a, 1b and 1c in FIG. 7. In FIG. 8, the abscissa indicates the flowing distance of the cooling water and the ordinate indicates the pressure of the cooling water.
In FIG. 8, polygonal lines A, B and C show the respective pressures of the cooling water caused to flow in the respective cooling plates 1a, 1b and 1c. The respective cooling water pressures decrease in the section from a branching point L of the branched pipes 2a, 2b and 2c to the respective inlets of the cooling plates 1a, 1b and 1c on the polygonal lines A, B and C because of pressure losses caused by the arrangement of the branched pipes. However, the pressures within these portion are represented by horizontal lines because the reductions of the pressures are negligible when compared with the pressures losses in the cooling water paths of the plates.
The pressures of the cooling water become A.sub.1, B.sub.1 and C.sub.1 at the respective inlets of the cooling plates 1a, 1b and 1c. After the cooling water has passed through the cooling water paths in the cooling plates, the cooling water pressures becomes A.sub.2, B.sub.2, and C.sub.2 at the respective outlets of the cooling plates. The pressure is P.sub.c at a flowing distance M in the outlet cooling main pipe 22 in which the cooling water is joined and through which the cooling water is discharged.
If the saturation pressure of the cooling water corresponding to the running temperature of the fuel cell is set to be P.sub.S1 as shown in FIG. 8, the cooling plate 1a disposed at the highest position starts to boil in the vicinity of the inlet, while the boiling point moves downwards as the height of the cooling plate decreases. For example, the boiling point in the cooling plate 1c moves to a position C.sub.2 in the vicinity of the outlet of the cooling plate. Accordingly, in the cooling plate 1c disposed at the lowest position, the rate of vapor caused by boiling is reduced so as to lower the cooling performance.
If the saturation pressure is set to be P.sub.S2 in order to increase the rate of vaporization by making the starting point of boiling approach a position in the vicinity of the inlet of the cooling plate 1c disposed at the lowest position, the cooling water pressures in the pipe 2a of the cooling plate 1a and the manifold portion connected with the pipe 2a are subjected to a pressure level P.sub.S2 so that the cooling water is likely to evaporate and flow into the cooling plate 1a through the pipe 2a in the form of a two-phase flow. However, the cooling water pressures in the pipes 2b and 2c of the respective cooling plates 1b and 1c are above P.sub.S2, the water flows in the liquid phase into the cooling plates 1b and 1c. The boiling condition in the cooling plate 1a is different from those in the cooling plates 1b and 1c because of the difference of the state of flow, so that the rate of vaporization is different among the cooling plates and produces differences in cooling performance thereamong.